The present invention relates to an apparatus for forming a silicon oxide film and a method of forming a silicon oxide film.
For example, in production of a MOS type semiconductor device, it is required to form a gate oxide film composed of a silicon oxide film on a surface of a silicon semiconductor substrate. In production of a thin film transistor (TFT), it is also required to form a gate oxide film composed of a silicon oxide film on a surface of a silicon layer formed on an insulation substrate. It can safely be said that reliability of the semiconductor devices depends upon these silicon oxide films. The silicon oxide films are therefore constantly required to have high dielectric breakdown durability and long-term reliability.
With a decrease in thickness of a gate oxide film and an increase in diameter of a substrate, an apparatus for forming a silicon oxide film has been being converted from a horizontal-type apparatus in which a process chamber (oxidation chamber) formed of quartz extends in a horizontal direction to a vertical-type apparatus in which a process chamber extends in a vertical direction. The reason therefor is as follows. Not only the vertical-type apparatus for forming a silicon oxide film can easily cope with an increase in a diameter of a substrate as compared with the horizontal-type apparatus, but also the vertical-type apparatus can serve to decrease formation of a layer of silicon oxide (to be referred to as xe2x80x9cnatural oxidexe2x80x9d hereinafter) caused by atmosphere taken into the process chamber of the vertical-type apparatus during transfer of silicon semiconductor substrates into the process chamber. However, even the use of the vertical-type apparatus for forming a silicon oxide film results in the formation of a natural oxide having a thickness of approximately 2 nm on the surface of the silicon semiconductor substrate. The natural oxide contains a large amount of impurities derived from atmosphere, and the presence of the natural oxide is not at all negligible when a gate oxide is decreased in thickness. There have been therefore proposed methods for preventing the formation of the natural oxide to the lowest level possible, such as (1) a method in which a nitrogen gas atmosphere is formed in a substrate transfer portion provided in a vertical-type apparatus by flowing a large volume of nitrogen gas (nitrogen gas purge method), and (2) a method in which a substrate transfer portion is vacuumed and then nitrogen gas or the like is introduced into the substrate transfer portion to discharge atmosphere (vacuum loadlock method).
Thereafter, in a state where an inert gas atmosphere is formed in the process chamber (oxidation chamber), silicon semiconductor substrates are brought into the process chamber (oxidation chamber). Then, an atmosphere of the process chamber (oxidation chamber) is replaced with an oxidative atmosphere and the silicon semiconductor substrates are thermally oxidized to form gate oxide films. For the formation of the gate oxide film, there is generally employed a method in which high-purity water vapor is introduced into the process chamber maintained at a high temperature to thermally oxidize the surface of the silicon semiconductor substrate (wet oxidation method). In this method, a gate oxide film having high electric reliability can be obtained as compared with a method in which the surface of the silicon semiconductor substrate is oxidized with high-purity dry oxygen gas (dry oxidation method). Included in the above wet oxidation method is a pyrogenic oxidation method (also called xe2x80x9chydrogen gas combustion oxidation method or wet oxidationxe2x80x9d) in which hydrogen gas is mixed with oxygen gas at a high temperature and is combusted and the so-generated water vapor is used. The pyrogenic oxidation method is widely used. In the pyrogenic oxidation method, generally, oxygen gas is supplied into a combustion chamber which is disposed outside the process chamber (oxidation chamber) and which interior is maintained at 700 to 900xc2x0 C., and then hydrogen gas is supplied into the combustion chamber to combust the hydrogen gas at a high temperature. The so-obtained water vapor is used as oxidizing species.
FIG. 21 shows a schematic view of a vertical-type apparatus for forming a silicon oxide film by the pyrogenic oxidation method. The vertical-type apparatus comprises a double-tubular structured process chamber 10 made of quartz and held perpendicularly, a water vapor inlet port 12 for introducing water vapor and the like into the process chamber 10, a gas exhaust portion 13 for exhausting the gas from the process chamber 10, a heater 14 for maintaining the interior of the process chamber 10 at a predetermined ambient temperature through a cylindrical heat equalizer tube 16 made of SiC, a substrate transfer portion 20, a gas introducing portion 21 for introducing nitrogen gas into the substrate transfer portion 20, a gas exhaust portion 22 for exhausting the gas from the substrate transfer portion 20, a shutter 15 for partitioning the process chamber 10 and the substrate transfer portion 20, and an elevator unit 23 for bringing silicon semiconductor substrates into and out of the process chamber 10.
A base portion 26 is attached to the elevator unit 23, and a heat insulation member 25 is disposed on the base portion 26. Further, onto the heat insulation member 25 is attached a substrate receiving member 24 made of quartz or SiC for receiving silicon semiconductor substrates therein. A sealing member 27 formed of, for example, an xe2x80x9cO-ringxe2x80x9d is attached to a marginal portion of the upper surface of the base portion 26, and when the substrate receiving member 24 is brought into the process chamber 10, the lower portion of the process chamber 10 is sealed with the base portion 26 (see FIG. 22). The base portion 26 is structured so as to flow coolant inside.
The heat insulation member 25 is also called a heat-retaining cylinder or a heat barrier, and generally, it is a hollow and cylindrical member having its top and bottom surfaces closed and being formed of quartz, and it has a structure in which the hollow portion is filled with glass fiber. Further, a piping 17 for flowing coolant is disposed outside the process chamber 10 and near the heat insulation member 25. In the above structure, damage of the sealing member 27 caused by radiation heat directly conducted to the base portion 26 in the process chamber 10, can be prevented and malfunction of the elevator unit 23 can be reliably prevented.
Hydrogen gas supplied to a combustion chamber 30 is mixed with oxygen gas at a high temperature and combusted in the combustion chamber 30 to generate water vapor. The water vapor is introduced into the process chamber 10 through a piping 31, a gas flow passage 11 and a water vapor inlet port 12. The gas flow passage 11 corresponds to a space between an inner wall and an outer wall of the double-tubular structured process chamber 10.
A conventional method of forming a silicon oxide film with a conventional apparatus having the above structure will be outlined with reference to FIGS. 23 to 25 hereinafter.
[Step-10]
First, nitrogen gas is introduced into the process chamber 10 through a piping 32, the combustion chamber 30, the piping 31, the gas flow passage 11 and the water vapor inlet port 12 to form a nitrogen atmosphere in the process chamber 10, and the ambient temperature in the process chamber 10 is maintained at 700 to 750xc2x0 C. with the heater 14 through the heat equalizer tube 16. The purpose in maintaining the ambient temperature in the process chamber 10 at the above temperature range is to decrease thermal shock which silicone semiconductor substrates 50 suffer when the silicon semiconductor substrates 50 are transferred into the process chamber 10. In this state, the shutter 15 is kept closed. The substrate transfer portion 20 is in a state where it is open to atmosphere. Further, the piping 17 has coolant flowing.
[Step-20]
Silicon semiconductor substrates 50 are transferred into the substrate transfer portion 20, and placed in the substrate receiving member 24. After the transfer of the silicon semiconductor substrates 50 into the substrate transfer portion 20 is completed, a door (not shown) is closed. Then, nitrogen gas is introduced into the substrate transfer portion 20 through the gas introducing portion 21 and is exhausted through the gas exhaust portion 22, to form a nitrogen gas atmosphere in the substrate transfer portion 20 (see FIG. 23A). The base portion 26 has coolant flowing inside.
[Step-30]
When a sufficient nitrogen gas atmosphere is formed in the substrate transfer portion 20, the shutter 15 is opened (see FIG. 23B), and the elevator unit 23 is actuated to elevate the substrate receiving member 24 approximately at a rate of 50 mm/minute, whereby the silicon semiconductor substrates 50 are transferred into the process chamber 10 (see FIG. 24A). When the elevator unit 23 reaches its uppermost position, the sealing member 27 comes into contact with the bottom of the process chamber 10, and the process chamber 10 is closed with the base portion 26, whereby the process chamber 10 and the substrate transfer portion 20 are no longer communicated with each other (see FIG. 22).
[Step-40]
Then, after the ambient temperature in the process chamber 10 is fully stabilized, the ambient temperature is increased up to 800 to 900xc2x0 C. (see FIG. 24B). Oxygen gas and hydrogen gas are supplied to the combustion chamber 30 through the pipings 32 and 33, and the hydrogen gas is mixed with the oxygen gas at a high temperature and combusted in the combustion chamber 30 to generate water vapor. The water vapor is introduced into the process chamber 10 through the piping 31, the gas flow passage 11 and the water vapor inlet port 12, and is exhausted through the gas exhaust portion 13 (see FIG. 25A), whereby a silicon oxide film is formed on the surface of each silicon semiconductor substrate 50. The temperature in the combustion chamber 30 is maintained at 700 to 900xc2x0 C., for example, with a heater (not shown).
[Step-50]
After the silicon oxide films having a predetermined thickness are formed, the supply of the water vapor into the process chamber 10 is terminated, and an inert gas atmosphere such as a nitrogen gas atmosphere is formed in the process chamber 10. Then, the ambient temperature in the process chamber 10 is decreased to 700 to 750xc2x0 C. for decreasing thermal shock on the silicon semiconductor substrates 50 (see FIG. 25B). Then, after the ambient temperature in the process chamber 10 is stabilized, the elevator unit 23 is actuated to lower the substrate receiving member 24, and the silicon semiconductor substrates 50 are transferred out of the substrate transfer portion 20.
Since coolant is continuously flowed in the piping 17 and further since coolant is continuously flowed inside the base portion 26, a large temperature gradient is caused between the ambient temperature in a process chamber area where the substrate receiving member 24 is positioned and the heat insulation member 25 when the ambient temperature in the process chamber the heat insulation member 25 has a surface (outer surface) temperature of 150 to 200xc2x0 C. or lower although it differs depending upon an apparatus for forming a silicon oxide film.
In the conventional method of forming a silicon oxide film, the ambient temperature in the process chamber 10 is decreased to 700 to 750xc2x0 C. and then the silicon semiconductor substrates 50 are transferred out of the process chamber 10 in Step-50. Therefore, even if dew (water drops) is formed on the surface of the heat insulation member 25 in the process of forming a silicon oxide film, the dew on the heat insulation member 25 is evaporated since the process chamber 10 is maintained to have an inert gas atmosphere at 700 to 750xc2x0 C. for a certain period of time in Step-50.
In recent years, the thickness of a gate oxide film is decreased for higher integration of an LSI, and with a decrease in the above thickness, the thermal oxidation temperature of the silicon semiconductor substrates is decreased. That is because the time period for the oxidation needs to be extremely decreased or shortened at a conventional thermal oxidation temperature of 800 to 900xc2x0 C.
Meanwhile, it has come to be known that when the thermal oxidation temperature is set at a low temperature (for example, 700 to 750xc2x0 C. or lower), the heat insulation member 25 has a surface temperature of less than 100xc2x0 C. so that dew (water drops) is formed on the surface of the heat insulation member 25 in the step of forming a silicon oxide film. When the silicon semiconductor substrates 50 are transferred out of the process chamber 10 with the dew on the surface of the heat insulation member 25, a metal portion or a metal member of the elevator unit 23 may be corroded. When the metal portion or the metal member is corroded, not only the elevator unit 23 malfunctions, but also a corroded portion can be a source which gives metal impurities. When metal impurities are therefore included in the process chamber 10, characteristics of the silicon oxide films are deteriorated. Further, since the silicon semiconductor substrates 50 have a temperature of hundreds degree C (xc2x0 C.) immediately after they are transferred to the substrate transfer portion 20, the dew on the surface of the heat insulation member 25 is evaporated to generate water vapor. When the water vapor comes into contact with the silicon semiconductor substrates 50, the silicon semiconductor substrates 50 suffer stains similar to water marks on their surfaces, which results in in-plane non-uniformity of the silicon oxide films.
It is therefore an object of the present invention to provide an apparatus for forming a silicon oxide film and a method of forming a silicon oxide film, in which a metal portion of the apparatus is not corroded with water, and which are free from the problem of in-plane non-uniformity of the silicon oxide film caused by stains similar to water marks on the surface of the silicon semiconductor substrate.
The apparatus for forming a silicon oxide film, provided by the present invention for achieving the above object, is an apparatus which has a process chamber and is for thermally oxidizing a surface of a silicon layer by introducing water vapor into the process chamber, and which further has dew-formation prevention/evaporation means for preventing dew formation in the process chamber and/or evaporating dew in the process chamber.
According to a first aspect of the present invention for achieving the above object, there is provided a method of forming a silicon oxide film, which method uses an apparatus having a process chamber and dew-formation prevention/evaporation means for preventing dew formation in the process chamber and/or evaporating dew in the process chamber, and in which a substrate having a silicon layer is transferred into the process chamber and water vapor is introduced into the process chamber to thermally oxidize a surface of the silicon layer, the method comprising thermally oxidizing the surface of the silicon layer in the process chamber, then replacing an atmosphere in the process chamber with inert gas in a state where no dew is formed in the process chamber and/or dew in the process chamber is evaporated, to remove water vapor out of the process chamber, and then transferring the substrate out of the process chamber. The replacement of the atmosphere in the process chamber with the inert gas may be initiated after there is brought a state where no dew is formed in the process chamber and/or dew in the process chamber has been evaporated, the above replacement may be initiated when the above state is brought, the above replacement may be initiated before the above state is brought, or the above replacement may be initiated in a state where dew is formed in the process chamber.
The apparatus for forming a silicon oxide film, provided by the present invention, or the apparatus suitable for practicing the method of forming a silicon oxide film according to the first aspect of the present invention further has;
(a) a water vapor generating apparatus,
(b) a water vapor inlet port disposed in an upper portion of the process chamber, for introducing water vapor into the process chamber,
(c) a gas exhaust portion disposed in a lower portion of the process chamber, for exhausting gas from the process chamber,
(d) a substrate transfer portion disposed below the process chamber and allowed to be communicated with the process chamber,
(e) a substrate receiving unit composed of a substrate receiving member for receiving a plurality of the substrates having silicon layers and a heat insulation member disposed on the bottom of the substrate receiving member,
(f) an elevator unit for elevating the substrate receiving unit upwardly and downwardly to transfer the substrate receiving unit from the substrate transfer portion to the process chamber and from the process chamber to the substrate transfer portion, and
(g) a heater unit disposed outside the process chamber for heating the silicon layer,
and preferably, the dew-formation prevention/evaporation means is structured so as to prevent dew formation on the surface of the heat insulation member and/or to evaporate dew on a surface of the heat insulation member. Preferably, the heat insulation member is controlled to have a surface temperature in a temperature range of from at least 100xc2x0 C., preferably up to 150xc2x0 C., with the dew-formation prevention/evaporation means. The heat insulation member is controlled to have a surface temperature in a temperature range of from at least 100xc2x0 C., preferably up to 150xc2x0 C., at least before the substrates are transferred out of the process chamber. The essence is that dew formation in the process chamber can be prevented and/or dew in the process chamber can be evaporated by maintaining the surface temperature of the heat insulation member in the above temperature range. That is, the surface of the heat insulation member may be controlled to have a surface temperature in the above temperature range before the formation of the silicon oxide film and continuously controlled to have the above temperature range during the formation of the silicon oxide film and immediately before the substrates are transferred out of the process chamber. Or, the surface of the heat insulation member may be controlled to have a surface temperature in the above temperature range when the formation of the silicon oxide film is initiated or during the formation of the silicon oxide film and continuously controlled to have a surface temperature in the above temperature range immediately before the substrates are transferred out of the process chamber. Or, the surface of the heat insulation member may be controlled to have a surface temperature in the above temperature range from completion of the formation of the silicon oxide film to a time immediately before the substrates are transferred out of the process chamber.
The dew-formation prevention/evaporation means preferably comprises an inert gas source, an inert gas inlet port disposed in the process chamber, a piping for connecting the inert gas inlet port and the inert gas source, and heating means for heating the inert gas to be introduced into the process chamber. In this case, preferably, the inert gas inlet port is arranged such that flow of the inert gas introduced into the process chamber collides with the heat insulation member. Further, preferably, the dew-formation prevention/evaporation means is provided with means (to be referred to as xe2x80x9cmoisture content measurement meansxe2x80x9d hereinafter) for measuring a moisture (water) content of the gas exhausted from the gas exhaust portion, and after the gas exhausted from the gas exhaust portion has a moisture content equal to, or smaller than, a predetermined moisture content, the substrates are preferably transferred out of the process chamber. The above inert gas can be selected from nitrogen gas, argon gas or helium gas. The moisture content measurement means includes a known humidity sensor and a dew-point hygrometer. If the flow of the inert gas introduced into the process chamber collides directly with the silicon layer, the silicon oxide films may have a deviation in thickness or in-plane non-uniformity in film thickness. When the gas exhausted through the gas exhaust portion has a temperature higher than the gas temperature which is measurable with the moisture content measurement means, gas cooling means may be disposed between the gas exhaust portion and the moisture content measurement means. The introduction of the inert gas into the process chamber may be initiated during the transfer of the substrates into the process chamber, the above introduction may be initiated before the formation of the silicon oxide film, the above introduction may be initiated concurrently with the initiation of formation of the silicon oxide film, the above introduction may be initiated during the formation of the silicon oxide film, or the above introduction may be initiated after completion of formation of the silicon oxide film. Further, the introduction of the inert gas into the process chamber can be terminated immediately before, during or after, the transfer of the substrates out of the process chamber.
Alternatively, the dew-formation prevention/evaporation means may comprise an auxiliary heating unit which is disposed outside the process chamber and is for heating the heat insulation member. In this case, the dew-formation prevention/evaporation means may further comprise temperature detecting means for detecting a surface temperature of the heat insulation member and a control unit for controlling the auxiliary heating unit. The above auxiliary heating unit may include a heater, or a piping and a hot medium flowing in the piping. The heater may be a heater according to a resistance heating method or a heater according to lamp heating method. The above temperature detecting means includes a thermocouple attached to, or integrated into, the surface of the heat insulation member, while it may be a temperature detecting means according to any method. The lower portion of the heat insulation member is more easily cooled than the upper portion thereof, and dew is liable to be formed on the surface of the lower portion. It is therefore preferred to attach or integrate the temperature detecting means to or into the surface of the lower portion of the heat insulation member. The operation of the auxiliary heating unit may be initiated before the formation of the silicon oxide film, or the operation of the auxiliary heating unit may be initiated concurrently with the initiation of formation of the silicon oxide film, the operation of the auxiliary heating unit may be initiated during the formation of the silicon oxide film, or the operation of the auxiliary heating unit may be initiated after completion of formation of the silicon oxide film. The operation of the auxiliary heating unit can be terminated immediately before, during or after, the transfer of the substrates out of the process chamber.
In the apparatus for forming a silicon oxide film, provided by the present invention, the water vapor generating apparatus can be at least one apparatus selected from;
(A) an apparatus which generates the water vapor by reacting hydrogen gas and oxygen gas at a high temperature,
(B) an apparatus which generates the water vapor by heating pure water,
(C) an apparatus which generates the water vapor by bubbling hot pure water with oxygen gas or inert gas,
(D) an apparatus which generates the water vapor by reacting hydrogen gas and oxygen gas in the presence of a catalyst, and
(E) an apparatus which generates the water vapor by a reaction between oxygen plasma and hydrogen plasma.
In the method of forming a silicon oxide film according to the first aspect of the present invention, preferably, the water vapor to be introduced into the process chamber is preferably generated (A) by reacting hydrogen gas and oxygen gas at a high temperature, (B) by heating pure water, (C) by bubbling hot pure water with oxygen gas or inert gas, (D) by reacting hydrogen gas and oxygen gas in the presence of a catalyst, or by a reaction between oxygen plasma and hydrogen plasma. For generating the water vapor, the above methods may be used alone or in combination.
The method of forming a silicon oxide film according to a second aspect of the present invention for achieving the above object comprises the steps of transferring a substrate having a silicon layer into a process chamber, then, introducing water vapor into the process chamber to thermally oxidize a surface of the silicon layer, then, replacing an atmosphere in the process chamber with an inert gas atmosphere to remove the water vapor and/or dew out of the process chamber, and transferring the substrate out of the process chamber.
In the method of forming a silicon oxide film according to the second aspect of the present invention, preferably, the inert gas exhausted out of the process chamber is measured for a moisture content when the water vapor and/or dew are removed out of the process chamber, and after the inert gas exhausted out of the process chamber has a moisture content equal to, or smaller than, a predetermined value, the substrate is transferred out of the process chamber. The above predetermined value can be a value at which dew is not formed on a transfer unit such as the elevator unit and the like when the substrate is transferred out with the transfer unit. That is, it can be a value equal to, or smaller than, a saturated water vapor pressure in atmosphere outside the process chamber. For example, preferably, it is 0.02 kg per kilogram of dry inert gas, while the predetermined value shall not be limited thereto. Further, preferably, the ambient temperature of the process chamber when the surface of the silicon layer is thermally oxidized and the ambient temperature of the process chamber when the water vapor and/or dew are removed out of the process chamber are nearly equal to each other, and in this case, the ambient temperature of the process chamber for the thermal oxidation of the surface of the silicon layer is preferably 750xc2x0 C. or lower. The above xe2x80x9cnearly equalxe2x80x9d implies not only that the above two ambient temperatures are exactly the same but also that the above two ambient temperatures may be different to some extent (for example, by about 20xc2x0 C.). Hereinafter, xe2x80x9cnearly equalxe2x80x9d will imply as explained above. When the ambient temperature of the process chamber for and during the thermal oxidation of the surface of the silicon layer is set at a value as explained above, a demand for a decrease in the thickness of the silicon oxide film can be satisfied, and thermal shock on the substrates can be decreased.
In the method of forming a silicon oxide film according to the first or second aspect of the present invention, the water vapor may be in a state where it is entrained by oxygen gas, air or inert gas such as nitrogen gas, argon gas, helium gas or the like when the water vapor is introduced into the process chamber.
Further, when the surface of the silicon layer is thermally oxidized, the ambient atmosphere of the process chamber may contain halogen element. In this case, there can be obtained the silicon oxide film excellent in time-zero dielectric breakdown (TZDB) characteristic and time-dependent dielectric breakdown (TDDB) characteristic. The halogen element can be selected from chlorine, bromine and fluorine, and of these, chlorine is preferred. The halogen element may be in the form of, for example, hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr or NF3. The content of the halogen element in the ambient atmosphere is 0.001 to 10% by volume, preferably 0.005 to 10% by volume, more preferably 0.02 to 10% by volume, based on the form of a molecule or a compound. For example, when hydrogen chloride gas is used, the content of the hydrogen chloride gas is preferably 0.02 to 10% by volume.
In the method of forming a silicon oxide film according to the first aspect of the present invention, there may be employed the steps of thermally oxidizing the surface of the silicon layer in the process chamber, then replacing an atmosphere of the process chamber with inert gas in a state where no dew is formed in the process chamber and/or dew in the process chamber is evaporated, to remove water vapor out of the process chamber, then, providing an inert gas atmosphere containing halogen element in the process chamber, thereby heat-treating the formed silicon oxide film, and then transferring the substrates out of the process chamber. The replacement of the atmosphere of the process chamber with the inert gas may be initiated after the process chamber has a state where no dew is formed and/or dew in the process chamber is evaporated, concurrently with a time when the process chamber has the above state, before the process chamber has the above state, or while the process chamber has a state where dew is present. Hereinafter, the time when the replacement of the atmosphere of the process chamber with the inert gas is initiated will be also as described above. In some case, after the surface of the silicon layer is thermally oxidized in the process chamber, there may be employed steps of replacing the atmosphere of the process chamber with inert gas in a state where no dew is formed in the process chamber and/or dew in the process chamber is evaporated, to remove water vapor out of the process chamber, then transferring the substrate out of the process chamber, then transferring the substrate into the process chamber again, and providing an inert gas atmosphere containing halogen element in the process chamber to heat-treat the formed silicon oxide film. Alternatively, after substrate is transferred out of the process chamber, there may be employed steps of transferring the substrate into a heat treatment apparatus and providing an inert gas atmosphere containing halogen element in the heat treatment apparatus to heat-treat the formed silicon oxide film.
In the method of forming a silicon oxide film according to the second aspect of the present invention, after the water vapor and/or dew in the process chamber are removed, there may be employed steps of providing an inert gas atmosphere containing halogen element in the process chamber to heat-treat the formed silicon oxide film, and then transferring the substrate out of the process chamber. Alternatively, after the substrate is transferred out of the process chamber, there may be employed steps of transferring the substrate into a heat treatment apparatus and providing an inert gas atmosphere containing halogen element in the heat treatment apparatus to heat-treat the formed silicon oxide film. When the silicon oxide film is heat-treated in the inert gas atmosphere containing halogen element, inter-lattice silicon atoms generated by the thermal oxidation of the silicon layer are diffused into a silicon crystal, and as a result, the interface state can be decreased. Further, due to a terminal effect of terminal bond site, removal of metal impurities and a dehydration effect of hydroxyl group, which are caused by a halogen atom, there can be obtained a silicon oxide film excellent in time-zero dielectric breakdown (TZDB) characteristic and time-dependent dielectric breakdown (TDDB) characteristic. The inert gas for the heat treatment includes nitrogen gas, argon gas and helium gas. Further, the halogen element includes chlorine, bromine and fluorine. Of these, chlorine is preferred. The form of the halogen element contained in the inert gas includes, for example, hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr and NF3. The content of the halogen element in the inert gas is 0.001 to 10% by volume, preferably 0.005 to 10% by volume, more preferably 0.02 to 10% by volume based on the form of a molecule or a compound. For example, when hydrogen chloride gas is used, the content of the hydrogen chloride gas in the inert gas is preferably 0.02 to 10% by volume.
In the method of forming a silicon oxide film according to the first aspect of the present invention, when the heat treatment is carried out in the process chamber subsequently to the thermal oxidation, desirably, the ambient temperature in the process chamber in the heat treatment is adjusted to 700 to 1200xc2x0 C., preferably to 700 to 1000xc2x0 C., more preferably to 700 to 950xc2x0 C. Or, the ambient temperature in the process chamber in the heat treatment is preferably adjusted so as to be nearly equal to the ambient temperature for the thermal oxidation of the surface of the silicon layer in the process chamber. In the method of forming a silicon oxide film according to the second aspect of the present invention, when the heat treatment is carried out in the process chamber subsequently to the thermal oxidation, the ambient temperature for the heat treatment in the process chamber is preferably adjusted so as to be nearly equal to the ambient temperature for the thermal oxidation of the surface of the silicon layer in the process chamber. In these cases, the time period for the heat treatment is 5 to 60 minutes, preferably 10 to 40 minutes, more preferably 20 to 30 minutes. Since the heat treatment is carried out in a state where no dew is formed in the process chamber, after dew in the process chamber is evaporated, or after water vapor and dew in the process chamber are removed, the formation of, for example, hydrochloric acid by a reaction of hydrogen chloride and water can be reliably prevented when the heat treatment is initiated.
In the method of forming a silicon oxide film according to the first or second aspect of the present invention, when the heat treatment is carried out in a heat treatment apparatus different from the apparatus for forming a silicon oxide film, the heat treatment may be carried out according to single wafer treatment, while it is preferably carried out according to furnace annealing treatment. In this case, the temperature for the heat treatment is 700 to 1200xc2x0 C., preferably 700 to 1000xc2x0 C., more preferably 700 to 950xc2x0 C. The time period for the heat treatment according to the furnace annealing treatment is 5 to 60 minutes, preferably 10 to 40 minutes, more preferably 20 to 30 minutes. The time period for the heat treatment according to the single wafer treatment is desirably 1 to 10 minutes.
The heat treatment may be carried out in a state where the inert gas atmosphere containing halogen element has a lower pressure than atmospheric pressure.
After the heat treatment, the silicon oxide film may be nitridation-treated. The nitridation treatment is preferably carried out in an atmosphere of N2O gas, NO gas or N2 gas. Of these, it is desirably carried out in an atmosphere of N2O gas. Otherwise, preferably, the nitridation treatment is carried out in an atmosphere of NH3 gas, N2H4 gas or hydrazine derivative, and then annealing treatment is carried out in an atmosphere of N2O gas or O2 gas. Desirably, the nitridation treatment is carried out at 700 to 1200xc2x0 C., preferably at 800 to 1150xc2x0 C., more preferably at 900 to 1100xc2x0 C., and in this case, the silicon layer is preferably heated by infrared irradiation or by furnace annealing treatment. Otherwise, the atmosphere for the heat treatment may be an atmosphere of nitrogen-containing gas. The nitrogen-containing gas includes N2, NH3, N2O, NO2 and NO.
When a MOS semiconductor device is produced from a silicon semiconductor substrate, conventionally, prior to the formation of a gate insulating film, the silicon semiconductor substrate is surface-cleaned by RCA cleaning in which it is cleaned with an NH4OH/H2O2 aqueous solution and further cleaned with an HCl/H2O2 aqueous solution, to remove fine particles and metal impurities from its surface. When the RCA cleaning is carried out, the surface of the silicon semiconductor substrate reacts with the cleaning liquid, to form a silicon oxide film having a thickness of approximately 0.5 to 1 nm. The so-formed silicon oxide film has a non-uniform thickness, and the components of the cleaning liquid remains in the silicon oxide film. The silicon semiconductor substrate is therefore immersed in a hydrofluoric acid aqueous solution to remove the above silicon oxide film, and then a chemical component is removed with pure water, whereby the silicon semiconductor substrate has a surface of which most part is terminated with hydrogen and a very small remaining part is terminated with fluorine. Obtaining a silicon semiconductor substrate surface of which most part is terminated with hydrogen and a very small remaining part is terminated with fluorine will be expressed as exposing the surface of the silicon semiconductor substrate in the present specification. Then, an insulating film, etc., are formed on the above surface of the silicon semiconductor substrate. Meanwhile, if the atmosphere before the formation of the insulating film, etc., is a high-temperature non-oxidizing atmosphere (for example, a nitrogen gas atmosphere), the surface of the silicon semiconductor substrate is roughened (has concave and convex portions formed thereon). The above phenomenon is said to be caused as follows. Part of Sixe2x80x94H bonds and/or Sixe2x80x94F bonds formed on the surface of the silicon semiconductor substrate by cleaning with a hydrofluoric aqueous solution and with pure water are eliminated by hydrogen and/or fluorine dissociation caused by an increase in temperature, and an etching phenomenon takes place on the surface of the silicon semiconductor substrate. For example, xe2x80x9cUltraclean ULSI Technologyxe2x80x9d, page 21, written by Tadahiro Ohmi, issued by Baifu-kan describes that when a silicon semiconductor substrate is temperature-increased to 600xc2x0 C. or higher in argon gas, the surface of the silicon semiconductor substrate is intensely roughened.
In the method of forming a silicon oxide film according to the first or second aspect of the present invention, it is preferred to initiate the thermal oxidation of the surface of the silicon layer at an ambient temperature at which silicon atoms are dissociated from the surface of the silicon layer for preventing the above phenomenon. Or, it is preferred to initiate the thermal oxidation of the surface of the silicon layer at a temperature of 500xc2x0 C. or lower, preferably 450xc2x0 C. or lower, more preferably 400xc2x0 C. or lower.
The ambient temperature at which silicon atoms are not dissociated from the surface of the silicon layer is preferably a temperature at which a bond between a terminal atom of the surface of the silicon layer and a silicon atom is not broken. In this case, the temperature at which silicon atoms are not dissociated from the surface of the silicon layer is preferably a temperature at which Sixe2x80x94H bonds on the surface of the silicon layer is not broken, or a temperature at which Sixe2x80x94F bonds on the surface of the silicon layer are not broken. When a silicon semiconductor substrate having an orientation (100) is used, each of most hydrogen atoms on the surface of the silicon semiconductor substrate is bound to each of two bonds of each silicon atom, and the surface of the silicon semiconductor substrate has a terminal structure of Hxe2x80x94Sixe2x80x94H. However, in a portion where the surface state of the silicon semiconductor substrate is broken (for example, step-formed portion), there is a terminal structure in which one hydrogen atom is bound to only one of two bonds of each silicon atom or a terminal state in which each of hydrogen atoms is bound to each of three bonds of each silicon atom. Generally, remaining bonds of each silicon atom are bound to silicon atoms inside a crystal. The term xe2x80x9cSixe2x80x94H bondxe2x80x9d in the present specification includes all of a terminal structure in which each hydrogen atom is bound to each of two bonds of each silicon atom, a terminal structure in which one hydrogen atom is bound to only one of two bonds of each silicon atom and a terminal structure in which hydrogen atom is bound to each of three bonds of each silicon atom. More specifically, the ambient temperature when the formation of the silicon oxide film on the surface of the silicon layer is 200xc2x0 C. or higher, preferably 300xc2x0 C. or higher, which is preferred in view of a throughput.
In the method of forming a silicon oxide film according to the first aspect of the present invention, the ambient temperature when the oxidation process is completed may be the same as the ambient temperature employed when the formation of the silicon oxide film is initiated, or the ambient temperature when the oxidation process is completed may be higher than the ambient temperature employed when the formation of the silicon oxide film is initiated. In the latter case, desirably, the ambient temperature when the oxidation process is completed is 600 to 1200xc2x0 C., preferably 700 to 1000xc2x0 C. or lower, more preferably 750 to 900xc2x0 C., while the above ambient temperature shall not be limited to these temperatures. Further, in the latter case, the method may include a first silicon oxide film formation step of initiating the thermal oxidation of the surface of the silicon layer at an ambient temperature at which silicon atoms are not dissociated from the surface of the silicon layer and then maintaining the atmosphere in an ambient temperature range in which silicon atoms are not dissociated from the surface of the silicon layer, for a predetermined period of time, to carry out the thermal oxidation, and a second silicon oxide film formation step of further thermally oxidizing the surface of the silicon layer at an ambient temperature higher than the ambient temperature range in which silicon atoms are not dissociated from the surface of the silicon layer, until the silicon oxide film having a desired thickness is obtained. Desirably, the temperature for the formation of the silicon oxide film in the second silicon oxide film formation step is 600 to 1200xc2x0 C. preferably 700 to 1000xc2x0 C. or lower, more preferably 750 to 900xc2x0 C. In the above first silicon oxide film formation step and the above second silicon oxide film formation step, one oxidation method may be employed, or different oxidation methods may be employed. In the first silicon oxide film formation step, in the second silicon oxide film formation step or in both the first and second silicon oxide film formation steps, the water vapor introduced into the process chamber may be entrained by an inert gas such as nitrogen, argon, helium. After the first silicon oxide film formation step is finished, there may be employed steps of increasing the ambient temperature in the process chamber, carrying out the second silicon oxide film formation step, replacing the atmosphere in the process chamber with inert gas in a state where no dew is formed in the process chamber and/or dew in the process chamber is evaporated, to remove water vapor from the process chamber, and then transferring the substrate out of the process chamber. Otherwise, there may be employed steps of carrying out the first silicon oxide film formation step in a first process chamber, then replacing the atmosphere in the first process chamber with inert gas in a state where no dew is formed in the first process chamber and/or dew in the first process chamber is evaporated, to remove water vapor from the first process chamber, then transferring the substrate out of the first process chamber, transferring the substrate into a second process chamber, carrying out the second silicon oxide film formation step in the second process chamber, then replacing the atmosphere in the second process chamber with inert gas in a state where no dew is formed in the second process chamber and/or dew in the second process chamber is evaporated, to remove water vapor from the second process chamber, and then transferring the substrate out of the second process chamber. In this case, there may be used an apparatus for forming a silicon oxide film having a structure in which one substrate transfer portion is commonly provided and two process chambers are provided above the substrate transfer portion, or there may be used two apparatuses for forming a silicon oxide film. In the latter case, preferably, the two apparatuses are connected with a vacuum transfer passage or a transfer passage filled with inert gas. In any case where the first silicon oxide film formation step and the second silicon oxide film formation step are carried out in one process chamber or in different process chambers, the heat treatment may be carried out after the second silicon oxide film formation step is carried out.
In the method of forming a silicon oxide film according to the second aspect of the present invention, the ambient temperature when the oxidation process is completed may be the same as the ambient temperature employed when the formation of the silicon oxide film is initiated, or the ambient temperature when the oxidation process is completed may be higher than the ambient temperature employed when the formation of the silicon oxide film is initiated. In the latter case, desirably, the ambient temperature when the oxidation process is completed is 750xc2x0 C. or lower, preferably 600 to 700xc2x0 C., while the above ambient temperature shall not be limited to these temperatures. Further, in the latter case, the method may include a first silicon oxide film formation step of initiating the thermal oxidation of the surface of the silicon layer at an ambient temperature at which silicon atoms are not dissociated from the surface of the silicon layer and then maintaining the atmosphere in an ambient temperature range in which silicon atoms are not dissociated from the surface of the silicon layer, for a predetermined period of time, to carry out the thermal oxidation, and a second silicon oxide film formation step of further thermally oxidizing the surface of the silicon layer at an ambient temperature higher than the ambient temperature range in which silicon atoms are not dissociated from the surface of the silicon layer, until silicon oxide films having a desired thickness are obtained. Desirably, the temperature for the formation of the silicon oxide film in the second silicon oxide film formation step is 750xc2x0 C. or lower, preferably 600 to 750xc2x0 C. In the above first silicon oxide film formation step and the above second silicon oxide film formation step, one oxidation method may be employed, or different oxidation methods may be employed. In the first silicon oxide film formation step, in the second silicon oxide film formation step or in both the first and second silicon oxide film formation steps, the water vapor introduced into the process chamber may be entrained by an inert gas such as nitrogen, argon, or helium. After the first silicon oxide film formation step is finished, there may be employed steps of increasing the ambient temperature in the process chamber, carrying out the second silicon oxide film formation step, removing the water vapor and/or dew from the process chamber, and then transferring the substrate out of the process chamber. Otherwise, there may be employed steps of carrying out the first silicon oxide film formation step in a first process chamber, then removing the water vapor and/or dew from the first process chamber, then transferring the substrate out of the first process chamber, transferring the substrate into a second process chamber, carrying out the second silicon oxide film formation step in the second process chamber, then removing the water vapor and/or dew from the second process chamber, and then transferring the substrate out of the second process chamber. In this case, there may be used an apparatus for forming a silicon oxide film having a structure in which one substrate transfer portion is commonly provided and two process chambers are provided above the substrate transfer portion, or there may be used two apparatuses for forming a silicon oxide film. In the latter case, preferably, the two apparatuses are connected with a vacuum transfer passage or a transfer passage filled with inert gas. In any case where the first silicon oxide film formation step and the second silicon oxide film formation step are carried out in one process chamber or in different process chambers, the heat treatment may be carried out after the second silicon oxide film formation step is carried out.
The silicon oxide film after the second silicon oxide film formation step can have a thickness as required depending upon a semiconductor device. The silicon oxide film after the first silicon oxide film formation step preferably has a thickness which is as small as possible. Silicon semiconductor substrates used for the production of semiconductor devices at present have a (100) crystal orientation in most cases, and however well the surface of the silicon semiconductor substrate is smoothened, a level difference called a step is necessarily formed on the (100) silicon surface. The step generally has a level difference by one layer of silicon atoms, while a level difference by 2 or 3 layers is sometimes formed. When a (100) silicon semiconductor substrate is used as a silicon layer, therefore, it is preferred that the silicon oxide film after the first silicon oxide film formation step should have a thickness of at least 1 nm, while the thickness shall not be limited thereto.
Before the silicon oxide film is formed on the silicon layer, generally, the surface of the silicon layer is cleaned by RCA cleaning in which the surface of the silicon layer is cleaned with an NH4OH/H2O2 aqueous solution and further cleaned with an HCl/H2O2 aqueous solution, to remove fine particles and metal impurities from its surface, and then the silicon layer is immersed in a hydrofluoric acid aqueous solution. However, if the silicon layer is then exposed to atmosphere, the surface of the silicon layer is contaminated, water or an organic substance may adhere to the surface of the silicon layer, or Si atoms in the surface of the silicon layer may bond to hydroxyl groups (OH) (for example, see xe2x80x9cHighly-reliable Gate Oxide Formation for Giga-Scale LSIs by using Closed Wet Cleaning System and Wet Oxidation with Ultra-Dry Unloadingxe2x80x9d, J. Yugami, et al., International Electron Device Meeting Technical Digest 95, pages 855-858). In the above case, if the formation of the silicon oxide film is initiated in the above state, a resulting silicon oxide film includes water and an organic substance or Sixe2x80x94OH, which may downgrade the characteristics of the silicon oxide film or may cause a defective portion. The defective portion refers to a portion of a silicon oxide film containing a silicon-dangling bond (Si.) or Sixe2x80x94H bond, or a portion of a silicon oxide film containing Sixe2x80x94Oxe2x80x94Si bond which is compressed due to a stress or has a bond angle different from that of Sixe2x80x94Oxe2x80x94Si in a thick or bulk silicon oxide film. For avoiding the above problem, therefore, it is preferred to include the step of cleaning the surface of the silicon layer before the formation of the silicon oxide film and carry out the step of forming the silicon oxide film without exposing the cleaned silicon layer to atmosphere (for example, by maintaining an inert gas atmosphere or a vacuum atmosphere in and from the cleaning of the surface of the silicon layer to the initiation of formation of the silicon oxide film). In this manner, there can be formed the silicon oxide film on that surface of the silicon layer which is terminated with hydrogen in most parts and terminated with fluorine in vary small remaining parts, and the downgrading of characteristics of the formed silicon oxide film or the occurrence of defective portions can be prevented.
In the method of forming a silicon oxide film according to the first or second aspect of the present invention, the silicon layer refers to a silicon layer on which a silicon oxide film is to be formed. The substrate having a silicon layer includes a silicon semiconductor substrate and the like. In this case, the silicon layer is also the silicon semiconductor substrate. Further, the substrate having a silicon layer includes a substrate on which an epitaxial silicon layer (including an epitaxial silicon layer formed by selective epitaxial growth method), a polycrystalline silicon layer, an amorphous silicon layer is formed, a substrate on which a silicon layer of an SOI structure is formed by a so-called laminating or SIMOX method, or a substrate having a silicon layer in which a semiconductor device or a semiconductor device element is formed. The silicon semiconductor substrate may be produced by any one of a CZ method, an MCZ method, a DLCZ method and an FZ method, and it may also be a silicon semiconductor substrate of which the crystal defect is removed beforehand by high-temperature hydrogen annealing method. When the silicon layer is a silicon semiconductor substrate itself, the silicon semiconductor substrate corresponds to the substrate and the silicon layer.
The method of forming a silicon oxide film, provided by the present invention, can be applied to the formation of silicon oxide films in various semiconductor devices such as the formation of a gate oxide film, a dielectric interlayer or an isolation region of a MOS type transistor, the formation of a gate oxide film of a top gate type or bottom gate type thin-film transistor, and the formation of a tunnel oxide film of a flash memory.
In the apparatus for forming a silicon oxide film or the method of forming a silicon oxide film according to the first aspect of the present invention, the dew-formation prevention/evaporation means is provided. In the method of forming a silicon oxide film according to the second aspect of the present invention, the substrates are transferred out of the process chamber after an inert gas atmosphere is provided in the process chamber and after water vapor and/or dew in the process chamber are removed. Therefore, metal members constituting the apparatus for forming a silicon oxide film are not corroded with water, and there can be prevented the problem that stains similar to water marks occur on a surface of a silicon layer and cause an in-plane non-uniformity in the thickness of the silicon oxide film. Particularly, when a silicon oxide film is formed at an ambient temperature at which silicon atoms are not dissociated from the surface of the silicon layer, the above problems are liable to occur, while the present invention can reliably prevent the occurrence of the above problems. Otherwise, when an inert gas atmosphere containing halogen atom is provided in the process chamber to heat-treat the formed silicon oxide film subsequently to the formation of the silicon oxide film in the process chamber, and if water remains in the process chamber, for example, hydrochloric acid is formed and it corrodes metal members constituting an apparatus for forming a silicon oxide film. In the present invention, the above phenomenon can be reliably prevented.